Discharge measurements

The stream’s velocity (flow) is modified by conditions along and around the stream, such as: 

  • Structures, such as dams and weirs, in the waterway; 
  • Removal (diversion) of water for use in irrigation, industry and households;
  • Rainfall, snow melt, and water releases from dams, power stations and industry;
  • Entry of groundwater;
  • Evaporation and evapotranspiration; and
  • Leakiness of the river bed and banks,

The size of a waterway and its flow rate affect its water quality. For example, discharges containing contaminants will have less effect on large swiftly flowing rivers than on small slow streams. This is one reason for measuring flow - to work out the load of contaminants and sediment the waterway is carrying.  Because discharge can have a significant effect on water quality, it is important that it is recorded at the time of sampling and, if possible, during the previous few days. It is particularly valuable to know if flows are at low, moderate or high level and if the level is rising or falling. This is because the concentrations of nutrients, turbidity and contaminants tend to be higher when the stream level is rising than when it is falling.  There are three ways to measure discharge. A simple method is to see how fast a floating object travels downstream over a chosen distance. This is called the float method. Secondly, flow data can be obtained from US Geological Service or the State Emergency Management Office, if your site is near a gauging station.  Thirdly, the velocity head rod (VHR) method can be used.  If your group has a flow-meter you should use the method recommended by USGS.

 

The float method is easy to understand and something most of us have done as children. You simply float an object on the water and measure the time it takes to travel a set distance.  The equipment you will need for this method includes: 

 
1.  Foam golf ball, float, organge peel etc,; Example:
2.  Net and/or person to catch the object;   Distance = 20 feet; Float time = 18 seconds
3.  Open-reel tape or survey rod; and a  Velocity = 20 ÷ 18 seconds = 1.11 feet/second
4.  Stopwatch

 

   

Procedure: Mark off a 20-foor length of the stream. Choose a section that is relatively straight and free of vegetation or obstacles. Avoid areas with a culvert or bridge because those structures will modify the true flow. If the flow is very slow, mark out a shorter distance. Position a person at each end of the section.  Place the ball on the surface near the middle of the stream at least two-feet or more up-stream of the end of the tape so it has time to come up to water speed.  When the ball is in line with the beginning of the tape, start the stopwatch. Stop the watch when the ball gets to the end of the section. Repeat the procedure at least three times at this site and average the results. To calculate the water velocity, divide the distance traveled in feet by the time taken in seconds.

 
US EPA’s Volunteer Manual recommends using a correction factor based on stream type.  The correction factor for rocky-bottom stream types is 0.8 and for sandy/muddy bottom stream it is 0.9.  These factors compensates for the variability in velocity and depth across the channel.  Thus, the example above now becomes 20 ÷ 18 = 1.11 ft/sec x 0.8 = 0.9 ft/sec.  Using a channel width of 10 feet and an average depth of 0.8 feet and the discharge calculation becomes 10 ft x 0.8 ft x 0.9 ft/sec = 7.2 cubic ft/sec (cfs) 
 

The velocity head rod (VHR) is a fast and inexpensive method of measuring velocity in a stream. The rod can be a yardstick or meter stick, or it can be made using a 3 - 6 foot long, very thin, piece of wood. A 26-gauge copper sheet is sometimes fastened to the cutting edge to protect it from abuse. Mark a scale in ½ inch increments on the rod, starting with zero at the bottom of the rod and stopping at 18 inches. Steps to find the flow rate with a velocity head rod are as follows:   

 

1.

Place rod in the water with sharp edge upstream. Measure stream depth on scale. 

2

Place rod sideways in the water. This will create turbulence and the water will "jump" or rise above its normal depth; velocity is proportional to this jump.  In most cases you cannot accurately measure a rise of < ¼ inch.

3

Measure depth of turbulent water next to rod.   Subtract stream depth from the turbulent depth reading to obtain the "jump height," or velocity head in inches.  

4

Find the stream velocity in feet per second from the table provided on the survey data sheet.

5.

Determine the stream velocity at intervals across the stream and average them to obtain the average stream velocity in feet per second.  The formula is: V = 8 x √R; where R is the rise in feet and V is velocity

 

Cross sectional shape varies with position in the stream, and discharge. The deepest part of channel occurs where the stream velocity is the highest. Both width and depth increase downstream because discharge increases downstream. As discharge increases the cross sectional shape will change, with the stream becoming deeper and wider.  The measurement of the cross section is necessary to determine the total discharge, which is the volume and velocity of the water.  Measuring the width and depth of the waterway, and multiplying these measurements together determine the cross section. The depth will vary across the stream and so the width and depth should be measured in small intervals and aggregated to determine the total area.   

 
To determine an average discharge, measure the depth in at least five positions across the stream, one of these positions being the deepest portion of the channel (thalweg).  In most cases the velocity and cross sectional area (width x depth) should be determined from a run.  Each time these measurements are made they should be completed from the same section of the reach. 

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